This invention relates to free electron lasers, and, more particularly, to self-seeded free electron lasers.
Free electron lasers (FELs) have existed for over a decade. FELs are broadly tunable laser sources and have the potential for high power outputs with average powers scalable to the megawatt level. But, thus far, the highest average power generated continuously by an FEL is only slightly more than 10 watts. The low average power is caused by a typically poor light extraction efficiency and by deleterious effects on the resonator mirrors, such as optical damage and thermally induced distortion. The extraction efficiency has been somewhat improved with the use of tapered magnetic field wigglers for the electron beam, but, due to losses in the conventional optical resonator, the efficiency of the FEL remains low, typically less than 1% for infrared wavelengths.
Four approaches to FELs have been tried or suggested: (1) oscillator, (2) amplifier, (3) externally-seeded amplifier, and (4) oscillator-amplifier. In an oscillator, relatively high reflectivity mirrors are used to have the light recirculate in an optical resonator. To attain high powers from the FEL, thus, requires an even higher recirculating power inside the resonator. The recirculating power is at least three times higher than the output power, and typically ten to twenty times higher than the output power. For a high-average power device, the damage to the mirror coatings and the requirements for cooling the mirrors add cost and operating complexity. Also, an oscillator has very tight tolerances on mirror alignment and length positioning, with concomitant problems in system stability and ease of use.
An amplifier does not have the problems with optics, since optics are not required. However, to attain saturation of the generated light, a much longer wiggler is required than with an oscillator, again with increased cost and system complexity. Another disadvantage of the amplifier is that the optical mode can grow larger than the wiggler gap and result in a loss of optical power.
An externally-seeded amplifier, i.e., an amplifier with externally generated and injected light to seed the generation of light, has the same basic constraints as an amplifier except that the output frequency is determined by the external light source. An external light source adds cost and complexity. Also, an external light source, e.g., a laser, may not be available to produce the desired optical frequency.
The oscillator-amplifier combines the advantages of an oscillator and amplifier. The oscillator is used to attain a moderate power level that is below the damage level of the optics. Also, the frequency stability is determined to a large degree by the resonator feedback. By the addition of a frequency selective device, the frequency can be tuned or precisely controlled, although such devices generally have much lower damage thresholds than for high-power optics, and consequently are difficult to use in even lower-power oscillators. The amplifier then is used to attain a high efficiency. This system is obviously much more complex than either the oscillator or amplifier configurations.
These issues are addressed by the present invention and a high gain amplifier is provided using a self seed injection-locked optical feedback to a wiggler to be further amplified by an input electron beam.
Accordingly, it is an object of the present invention to provide a FEL having a high output laser average power.
It is another object of the present invention to provide a high power FEL that is relatively low in complexity.
One other object of the present invention is provide a high power FEL operating in a stable regime.
Still another object of the present invention is to minimize damage to optical components in the laser output system.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.